Tilt cylinder anticavitation circuit

ABSTRACT

Cavitation in the tilt cylinder circuit for a lift truck is prevented by cross connecting one of the tilt cylinder supply lines with the return-to-reservoir line of another circuit in the lift truck hydraulic system. A cone-way check valve is incorporated in the cross connecting passage to prevent flow from the tilt cylinder circuit to the second circuit.

Unite States Patent 11 1 Matthews TILT CYLINDE-R ANTICAVITATION CIRCUITInventor: Dean A. Matthews, Homewood, Ill.

Allis-Chalmers waukee, Wis.

Filed: Jan. 20, 1972 Appl. No.: 219,414

Assignee: Corporation, .Mil-

U.S. Cl. ..214/674, 60/52 HE Int. Cl. ..B66f 9/20 Field of Search..2l4/672, 674, 673; 187/9;

References Cited UNITED STATES PATENTS 3/1956 Eames ....60/52 HE Mar.27, 1973 3,568,868 3/1971 Chichester ..2l4/674 Primary ExaminerAlbert J.Makay Assistant ExaminerLawrence J. Oresky Attorney-Charles L. Schwab etal.

Cavitation in the tilt cylinder circuit for a lift truck is ABSTRACTprevented by cross connecting one of the tilt cylinder supply lines withthe return-to-reservoir line of another circuit in the lift truckhydraulic system. A cone-way check valve is incorporated in the crossconnecting passage to prevent flow from the tilt cylinder circuit to thesecond circuit.

8 Claims, 2 Drawing Figures LOAD PAIENIEUIIARZYIQYS 3,722,723

45v \LIFT CYLINDER 23 \J- -L A l I 50 4I 37 T r T I I 42 TILT CYLINDERS4O 4.,

1. .L .L .L I I MAN 36 FLOW DIVIDEQ 32 46 VALVE J -39 47 49 POWERSTEERNG pUMp\ 3I 5\8 IE i y\ CYLINDER I 33' POWER STEERING VALVE u-FILTER TILT CYLINDER ANTICAVITATION CIRCUIT BACKGROUND OF THE INVENTIONVarious means have been employed to prevent cavitation in hydrauliccircuits. Heretofore it has been suggested that locking valves beinterposed between the tilt cylinder control valve and the tiltcylinders. These type valves prevent the flow of fluid to and from thetilt cylinders unless fluid under pressure is being supplied to the tiltcylinders. Another anticavitation system for tilt cylinders involves theuse of a check valve in the tilt cylinder control valve which permitsflow of fluid from the exhaust side of the valve to the expanding sideof the tilt cylinder.

BRIEF DESCRIPTION OF THE PRESENT INVENTION In the present invention theside of the tilt cylinder which tends to cavitate, under conditions oflow rate of flow from the source of fluid pressure, is cross connectedby a one-way circuit to a fluid supply being maintained at a higherpressure than available in the tilt cylinder circuit. If the operatorchooses to tilt the mast through operation of the tilt cylinder controlvalve when no fluid flow is being delivered to the tilt cylinder controlvalve, the load on the lift truck carriage may cause one side of thetilt cylinders to be exhausted of fluid faster than fluid can besupplied to the expanding side, thus giving rise to cavitation in theexpanding side.

In the illustrated embodiment of the invention a single pump suppliesfluid to a flow divider which gives priority of flow to the powersteering circuit. Low priority flow is provided to the lift cylinder andtilt cylinder circuits. Thus, at low engine speed the available fluidflow is directed to the power steering circuit and little or no flow isprovided to the lift cylinder and tilt cylinder circuits. If theoperator choose to tilt the mast through operation of the tilt cylindercontrol valve when the engine is running at a very low speed, and areasonably large load is being carried by the forks of the lift truck,it is possible for the load to force fluid from one end of the tiltcylinders faster than fluid can be supplied to their correspondingopposite ends. In the typical mast tilt arrangement, fluid would beexhausted from the rod end of the tilt cylinders and the closed end ofthe cylinders would then need to be supplied with fluid and if not sosupplied, cavitation will occur. As is well known in the art, cavitationoccurs when a partial vacuum exists in a hydraulic cylinder. The airpockets evolving from cavitation give rise to problems in controllingthe mast in the desired tilt condition, particularly when the load iselevated to a near maximum height and the truck is moved to adjustposition for depositing the load. In order to supply an adequatequantity of fluid to the closed end of the cylinders under theseconditions, applicant conceived of cross connecting the closed end ofthe cylinders with the return flow from the higher priority powersteering circuit. This insures an adequate flow of fluid to the closedend of the tilt cylinders when supply from the pump is inadequate tofill the displaced volume. A check valve is provided in the crossconnecting passage to prevent undesirable reverse flow.

It is an object of this invention to provide a tilt cylinderanticavitation circuit wherein the side of the tilt cylinder tending tocavitate is supplied with fluid from another circuit.

It is more specifically an object of the present invention to providethe side of the tilt cylinder tending to cavitate with fluid underpressure from the return side of the power steering circuit.

It is a further object of the present invention to provide a hydraulicsystem for a lift truck wherein a single pump supplies fluid to a tiltcylinder circuit and a power steering circuit through a flow dividerwhich gives priority to the power steering circuit and wherein the tiltcylinder circuit is interconnected through a oneway valve with thereturn flow from the power steering circuit in a manner preventingcavitation in the tilt cylinder circuit.

These and other objects and advantages of. the present invention will beobvious to those who are A familiar with the art when the followingdescription is read in conjunction with the drawings in which:

FIG. 1 is a side view of the lift truck of the type in which the presentinvention is incorporated; and,

FIG. 2 is a schematic illustration of a hydraulic system incorporationthe present invention. 7

Referring to FIG. 1 a fork lift truck 11 is illustrated wherein aninternal combustion engine 12 drives a pair of drive wheels 13 at thefront of the truck through a transmission 14. The rear of the truck issupported by a pair of steerable wheels 16 controlled by a manuallyoperated steering wheel l7.adjacent the operators seat 18. A mast 21 isconnected on a transverse axis by appropriate pivot means 22 and it istilted from the vertical position, in which illustrated, by a pair ofdoubleacting tilt cylinders 23, only one of which is shown in FIG. 1. Alift truck carriage 26 is supported on the mast 21 for verticalreciprocal movement thereon and has forks 27 thereon for supporting andcarrying load 28. The carriage 26 is elevated and lowered throughoperation of a conventional lift cylinder not shown in FIG. 1.

Referring to FIG. 2, which shows a hydraulic system for the lift truckillustrated in FIG. 1, a single positive displacement pump 31 suppliesfluid under pressure to a conventional flow divider valve 32 whichawards priority of flow to a steering valve 47 by way of a supplypassage 33 of a power steering circuit 35. Second priority flow isdirected by the flow divider valve 32 to a tilt control valve 34 througha supply conduit 36 of the tilt cylinder control circuit 38. A controlvalve 40 controls fluid flow to a lift cylinder 45 via circuit 37. Thus,it is seen that a single pump 31 supplies pressure fluid for theauxiliary or steering circuit 35, the lift circuit 37 and tilt circuit38. A single reservoir line 39 handles the return flow from all threecontrol valves 34, 40 and 47. The tilt cylinders 23 42, correspondingends interconnected by conduits 41, 2, which in turn are connected tothe tilt cylinder control valve 34 by tilt cylinder control conduits 50,43, respectively.

The power steering cylinder 46 is connected to the power steeringcontrol valve 47 by a pair of supply conpower steering circuit 35. Asthe output of the pump 31 increases with engine speed, the excess flowis directed to the control valves, 34, 40 through the supply conduit 36.At low engine or idle speed, it is possible to get very little or nofluid delivery to supply conduit 36. if at such time the operatordecides to tilt the mast 21 forwardly by operation of the tilt controlvalve 34, fluid will be exhausted from the rod ends of the tiltcylinders 23 by way of conduits 41, 50 and 39. The closed ends of thetilt cylinders 23 will be supplied fluid through conduits 36, 43, 42.However, at low engine speed, there will be little or no flow to theclosed ends of the tilt cylinders. Accordingly, a partial vacuum iscreated in the closed ends of the tilt cylinders 23 which, as is wellknown in the art, gives rise to cavitation.

Applicant conceived of providing a one-way flow means, in the form ofconduit 56 and check valve 57, between the exhaust passage 51 and thesupply conduit 43 connected to the closed ends of the tilt cylinders.The flow resistance of filter 52 insures a relatively high back pressurewhich assures a fairly rapid flow of make-up fluid to the closed ends tothe tilt cylinders 23 from the exhaust passage 51. Check valve 57prevents flow from the tilt cylinder supply conduit 43 to exhaustpassage 51. By joining the reservoir line 39 and the exhaust passage 51upstream of the filter 39, the return flow from the tilt cylinders isadded to the exhaust flow from the high priority auxiliary circuit 35,thus insuring adequate flow to avoid tilt cylinder cavitation even atlow engine speed. Preferably, the cross connecting conduit 56 isconnected as close as possible to the power steering valve 47 so as totake full advantage of the line pressure drop between the power steeringvalve 47 and the reservoir. If the filter does not afford sufficientresistance, a simple flow restrictor could be placed at point 58 in theexhaust passage 51 to artificially create the desired back pressure toinsure flow to the closed ends of the cylinders 23 whenever a cavitationcondition might exist.

From the foregoing description it is apparent that the present inventionsolves a tilt cylinder cavitation problem in an economical and effectivemanner.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In a lift truck having a tiltable mast with elevatable load carryingcarriage, the combination comprising:

a hydraulic tilt cylinder;

a hydraulic circuit for controlling said tilt cylinder including a tiltcontrol valve,

tilt cylinder supply conduits interconnecting said tilt control valveand said tilt cylinder and supply and exhaust conduits connected to saidtilt control valve;

an auxiliary hydraulic cylinder;

a hydraulic circuit for controlling said auxiliary cylinder includingcontrol valve means,

input and exhaust passages connected to said control valve means, and

supply passages interconnecting said auxiliary cylinder and said controlvalve means;

fluid supply means operable to supply fluid under pressure to said inputconduit and said input p i g i d an anticavltation circuit including across passage interconnecting one of said tilt cylinder supply conduitsand said exhaust passage including one-way flow control means in saidcross passage permitting flow to fluid therein only from said exhaustpassage to said one supply conduit.

2. The combination of claim 1 wherein said fluid supply means includes apump and a flow divider valve connected to the output side of said pumpand connected in fluid delivery relation to said input conduit and saidinput passage, said flow divider valve effecting priority of flow fromsaid pump to said input passage.

3. The combination of claim 2 wherein said control valve means is anopen center control valve.

4. The combination of claim 3 wherein said auxiliary hydraulic cylinderis a steering cylinder for said lift truck.

5. The combination of claim 1 wherein said fluid supply means effects apriority of fluid supply to said input passage.

6. The combination of claim 5 wherein said auxiliary hydraulic cylinderis a steering cylinder for said lift truck.

7. The combination of claim 6 wherein said control valve means is anopen center steering control valve.

8. The combination of claim 7 wherein said one-way flow control means isa check valve.

' i i i

1. In a lift truck having a tiltable mast with elevatable load carryingcarriage, the combination comprising: a hydraulic tilt cylinder; ahydraulic circuit for controlling said tilt cylinder including a tiltcontrol valve, tilt cylinder supply conduits interconnecting said tiltcontrol valve and said tilt cylinder and supply and exhaust conduitsconnected to said tilt control valve; an auxiliary hydraulic cylinder; ahydraulic circuit for controlling said auxiliary cylinder includingcontrol valve means, input and exhaust passages connected to saidcontrol valve means, and supply passages interconnecting said auxiliarycylinder and said control valve means; fluid supply means operable tosupply fluid under pressure to said input conduit and said inputpassage; and an anticavitation circuit including a cross passageinterconnecting one of said tilt cylinder supply conduits and saidexhaust passage including one-way flow control means in said crosspassage permitting flow to fluid therein only from said exhaust passageto said one supply conduit.
 2. The combination of claim 1 wherein saidfluid supply means includes a pump and a flow divider valve connected tothe output side of said pump and connected in fluid delivery relation tosaid input conduit and said input passage, said flow divider valveeffecting priority of flow from said pump to said input passage.
 3. Thecombination of claim 2 wherein said control valve means is an opencenter control valve.
 4. The combination of claim 3 wherein saidauxiliary hydraulic cylinder is a steering cylinder for said lift truck.5. The combination of claim 1 wherein said fluid supply means effects apriority of fluid supply to said input passage.
 6. The combination ofclaim 5 wherein said auxiliary hydraulic cylinder is a stEering cylinderfor said lift truck.
 7. The combination of claim 6 wherein said controlvalve means is an open center steering control valve.
 8. The combinationof claim 7 wherein said one-way flow control means is a check valve.